There are two eye drugs that have been declared equivalently miraculous. Tested side by side in six major trials, both prevent blindness in a common old-age affliction. Biologically, they are cousins. They’re even made by the same company.

But one holds a clear price advantage.

Avastin costs about $50 per injection.

Lucentis costs about $2,000 per injection.

Doctors choose the more expensive drug more than half a million times every year, a choice that costs the Medicare program, the largest single customer, an extra $1 billion or more annually.

Spending that much may make little sense for a country burdened by ever-
rising health bills, but as is often the case in American health care, there is a certain economic logic: Doctors and drugmakers profit when more-costly treatments are adopted.

Genentech, a division of the Roche Group, makes both products but reaps far more profit when it sells the more expensive drug. Although Lucentis is about 40 times as expensive as Avastin to buy, the cost of producing the two drugs is similar, according to scientists familiar with the drugs and the industry.

Doctors, meanwhile, may benefit when they choose the more expensive drug. Under Medicare repayment rules for drugs given by physicians, they are reimbursed for the average price of the drug plus 6 percent. That means a drug with a higher price may be easier to sell to doctors than a cheaper one. In addition, Genentech offers rebates to doctors who use large volumes of the more expensive drug.

“Genentech continues to maintain that Lucentis is the most appropriate medicine,” the company said in a statement, adding that it costs “significantly” more to make and is tailored for use in the eye. The drug “has made an immense impact.”

Many ophthalmologists, however, are skeptical that it provides any added value over the cheaper alternative.

“Lucentis is Avastin — it’s the same damn molecule with a few cosmetic changes,” said J. Gregory Rosenthal, a Toledo ophthalmologist who, outraged by the price, co-founded a group called Physicians for Clinical Responsibility to protest its use. “Yet Americans are paying a billion dollars every year for no good reason — unless you count making Genentech rich.”

The story of Genentech’s two drugs, Lucentis and Avastin, began with a scientific marvel — a breakthrough in biology that, thanks to the vast budgets of U.S. entitlement programs, has produced enormous financial returns.

Those profits have yielded benefits. By paying for such drugs without regard to cost, the Medicare system has helped stimulate investment in medical research that contributes to the development of more lifesaving technologies.

But the flow of cash also pushes up the health-care costs that are projected to deplete federal budgets. For while Genentech has aggressively marketed the more expensive drug and sought to restrict the use of the cheaper one, critics say, Medicare has been powerless to do anything but pay up.

That’s because over the past seven years, despite pleas from the Food and Drug Administration and doctors groups, Genentech has maintained the barriers that make it harder for doctors to use the cheaper drug.